Notebook computers traditionally take 2-3 hrs to fully charge. There are new Li-ion cells coming on the market that can accept higher charge currents than are available from standard AC adapters. Providing such higher charge currents given associated heat generation and electrical challenges is not a straightforward task. As the charge rate increases, the heat created by the charge circuits also increases.
Given that these devices are small and portable, it is desirable to keep this heat generation as low as possible to keep the devices in desired operating temperatures as well as to conserve energy.